Encyclopedia of Evolution.pdf - Online Reading Center

ence their intelligence. There is also a correlation between
IQ and body left-right asymmetry. Left-right asymmetry is
considered an indicator of stress and poor nutrition during
fetal development. Those who reject a genetic component to
intelligence (within the normal human range) interpret these
results to indicate that intrauterine stresses cause lower IQ.
Human intelligence evolved, but did its components?
Did neural networks for art, phobias, religion, and violence
evolve? Or did evolution produce a brain that was simply big
enough to develop these abilities and potentials? Evolutionary
science can, at this point, proceed no further than the
anatomy and physiology of brain science.
How Human Intelligence Evolved
One of the strongest factors that have caused some lineages
of animals to evolve higher intelligence than others is their
way of obtaining food.
• Among herbivores, intelligence is correlated with selectivity
of food. A great deal of plant tissue is toxic. Many herbivores
get around this problem by physiological means, such
as complex digestive systems (see coevolution). Others
get around the problem by means of intelligence. Howler
monkeys eat leaves; they have long digestive systems that
allow time for bacterial fermentation to break down many
of the toxins. In contrast, spider monkeys selectively eat
fruits and nuts, and have shorter digestive systems. Spider
monkeys have larger brains than howler monkeys. Their
relatively greater intelligence allows spider monkeys to
find more food energy, and the energy requirement of their
large brains requires them to do so. Among squirrels, the
size of the hippocampus (the portion of the brain used to
remember locations) is proportional to the amount of dispersion
they use in food storage. Gray squirrels store their
nuts more widely, and have a larger hippocampus, than red
squirrels. The hippocampus swells in size in squirrels in the
autumn, when food storage begins.
• Predators are usually more intelligent than herbivores,
because they must seek and chase prey. Scavengers may
also have higher intelligence than herbivores.
Brain size in human ancestors began to increase after the
evolution of upright walking (see bipedalism), and simultaneous
with the evolution of tool use. australopithecines walked
upright, but had brains no larger than those of chimpanzees,
and made no stone tools (although, like modern chimps, they
may have used sticks and rocks as tools). The first species of the
human genus (see Homo Habilis) had larger brains and made
more complex tools than the australopithecines. Both intelligence
and tools may have allowed early humans to be effective
scavengers and only much later to be effective hunters.
Once brain size had begun to increase, natural selection
reinforced the increase. Since brains are expensive, early
humans with larger brains needed more calories. There was
therefore an evolutionary advantage to increased intelligence
that would allow humans to select higher quality plant foods,
to more intelligently locate animals killed by larger predators,
and to hunt. This created a positive feedback situation in
which larger brains created a demand for even larger brains.
intelligence, evolution of
A continued increase in brain size was inconvenient in
hominins because of upright posture. Upright walking requires
a narrow pelvis, with the legs directly beneath. This arrangement
puts a constraint on the size of the birth canal. The head
is the largest part of the baby that must pass through this canal.
Therefore upright walking constrains the size of the head at
birth. Unless natural or other selection provided a considerable
advantage to a large brain, this would have placed an upper
limit on human brain size. What actually happened, however,
is that human babies began to be born at an earlier stage of
development. If human embryonic development continued until
the same stage that it does in other primates, human babies
would be born after 14 months of gestation, rather than nine
(see life history, evolution of). Human babies are still as
helpless as embryos when they emerge, which would endanger
their survival. The continued evolution of human intelligence
allowed mothers to care for even these helpless infants. It has
been suggested that the helplessness of the infant, and the preoccupation
of the mother with its care, created an advantage
for fathers to provide care and food for the mother and infant,
which fathers in other primate species seldom do. According to
this explanation, the evolutionary increase in brain size encouraged
the evolution of family units and of high intelligence. The
early birth of human babies, and high intelligence, are two features
of neoteny, which is the preservation of juvenile characteristics
into reproductive adulthood, and which characterizes
the modern human species more than any other.
Once high intelligence had begun to evolve in humans, it
would provide numerous advantages to individuals who possessed
a greater degree of it than other individuals:
• Intelligent individuals were able to exploit their environment
for resources more efficiently, as explained above.
• Intelligent individuals had a superior ability to understand
and manipulate social relationships and kinship ties, as is
observed in many primate societies. Called Machiavellian
intelligence after the Renaissance Italian political philosopher
Niccolò Machiavelli, such abilities would allow
their possessor to become a dominant male and sire a
large proportion of the offspring in the group. Dominant
males in primate groups are often those who are best able
to form alliances, rather than simply the strongest or the
best fighters.
• Intelligent individuals could gather and analyze information
about other human groups, the more effectively to
outwit and outfight them.
Human intelligence went far beyond the practical aspects
of hunting, gathering, fighting, and child care. Defense from
outside threats, efficiency at obtaining resources, and success
at gaining dominance within societies as explanations for the
evolution of human intelligence are very male-biased, as it is
primarily males that fight. What about female intelligence?
Furthermore, as evolutionary biologist Geoffrey Miller has
pointed out, Machiavellian intelligence is insufficient to explain
the vast range of human intellectual capabilities. For example:
• The human ability for language far exceeds what is necessary
for efficient communication during foraging and